Well, the U.S. couldn't do it until the Iraqi government gave U.S. soldiers immunity from prosecution, through what's called a "diplomatic note." If those U.S. soldiers committed any crimes or had any legal troubles while advising Iraqis, the U.S. wanted to handle any prosecutions.

Here's what Pentagon spokesman Rear Adm. John Kirby had to say today:

"Many of you have asked today about the status of legal protections for the small number of military advisors that will be working inside Iraq.

"I can confirm for you that Iraq has provided acceptable assurances on the issue of protections for these personnel via the exchange of diplomatic note. Specifically, Iraq has committed itself to providing protections for our personnel equivalent to those provided to personnel who were in country before the crisis. We believe these protections are adequate to the short-term assessment and advisory mission our troops will be performing in Iraq. With this agreement, we will be able to start establishing the first few assessment teams."

That was fast! Back in 2011, the U.S. plan to have thousands of U.S. military trainers in Iraq fell apart because there was no Status of Forces Agreement, which included the vital "immunity from prosecution." After many months of wrangling, the Iraqis wouldn't agree to it. So the U.S. just left. Now with Iraq under attack from ISIS, it's a different story.